1. Technical Field
This invention relates to digital television (DTV) and, more particularly, to optimization of television reception by selecting among, or combining, input signals from multiple antennas for mitigation of multipath.
2. Background Art
High definition television (HDTV) is an emerging technology that is capable of providing service either in an analog or digital format. In the United States, research on HDTV has focused on digital, rather than analog, technology. While digital HDTV is not currently available, Japanese companies have developed an HDTV system based on analog technology (known as Hivision) that has been in use since 1991. Because of the potential advantages of digital HDTV and many technical problems shared by both types of systems, research in digital HDTV has also been active in Japan. See, for example, David K. Kahaner in xe2x80x9cHDTV Research in Japanxe2x80x9d, IEEE Micro, October 1993, pp. 49-53.
One of the most important prevalent problems in digital television (DTV) is that of multipath. In fact, it is useful to think of the DTV channel as multipath limited and not power limited. Multipath may arise from fixed structures, such as building walls, acting as reflectors in the transmission channel. Moving objects, such as airplanes, may also cause a multipath condition. Even microreflections in cabling can cause multipath. See, for example, P. T. Marhiopoulos and M. Sablatash, xe2x80x9cDesign of a Ghost Canceling Reference Signal for Television Systems in North Americaxe2x80x9d, Proceedings of Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, Canada, 14-17 September 1993, pp. 660-663.
The effect of multipath is to create xe2x80x9cghostsxe2x80x9d in the displayed TV image. The statistics of multipath ghosts have been studied and compiled by, among others, the BTA (Japan""s Broadcasting Technology Association). A BTA survey reported that 92% of ghosts are within a xe2x88x924 to 26 xcexcsecond range, and when extended to xe2x88x924 to 37 xcexcseconds, almost all occasions of ghost creation are covered.
An adaptive equalizer has been proposed to xe2x80x9cundoxe2x80x9d the effects of the multipath. In its crudest form, an adaptive equalizer can be thought of as a signal processor that estimates the parameters of a hypothetical filter that best describes the channel. The signal processor adjusts the taps of the adaptive equalization filter to approximate an inverse of the hypothetical filter, thus inverting or undoing the effects of the multipath.
The BTA, and other concerns, designed a xe2x80x9cghost canceling reference (GCR)xe2x80x9d transmitted signal to mitigate these multipath induced effects. The BTA GCR was found to be less than satisfactory in some cases. While homes with outdoor antennas displayed non-varying (stationary) ghosting conditions which could be largely corrected, those homes with indoor antennas experienced changing (dynamic) ghosts. These ghosting conditions were more prevalent when people were moving about the room or other moving objects were in the signal path. The BTA ghost canceller generally was unable to adequately compensate for these conditions. In fact, false ghosts were actually added to an already ghosted picture, leading to reduced picture quality.
Thus, multipath behavior of the DTV channel is important for two different regimes, the outdoor antenna propagation channel and the indoor antenna propagation channel. The former is well-studied and understood. The latter regime still presents a problem. The chief difference is the presence of significant reflectors near the indoor receiving antenna, the presence of which implies that there will be multipath whose delay occasions it to fall within a symbol period. In order to resolve multipath differences of such limited extent, special techniques must be employed or the channel diagnostic signal must have a very wide effective bandwidth. According to S. Salous in xe2x80x9cIndoor and Outdoor UHF Measurements with a 90 MHZ Bandwidthxe2x80x9d, IEEE Colloquium on Propagation Characteristics and Related System Techniques for Beyond Line-of-Sight Radio, 1997, pp. 8/1-8/6, the extent of multipath delays of outdoor environments can be a few tens of xcexcseconds, whereas in indoor environments, it is on the order of a few hundred nanoseconds. While multipath components can be adequately resolved with a 10 to 40 MHZ bandwidth for outdoor environments, the resolution of multipath for indoor environments requires 90 to 100 MHZ bandwidth.
In order to receive weak television signals in rapidly changing mutlipath interference environments, such as encountered in a house which does not have an external (outdoor) television antenna, a plurality of indoor antennas are attached to a television receiver. These indoor antennas are located, constructed or oriented in a manner to provide a diversity of input signals to the television receiver. This diversity is such that if one antenna, or a particular combination of more than one antenna, does not provide an acceptable television signal, then another antenna, or a different combination of more than one antenna, may provide an acceptable television signal. In a preferred embodiment, the television receiver is provided with more than one television tuner, so that, while one tuner is receiving the signal being viewed from one antenna or a particular combination of more than one antenna, the second tuner scans the signals from all other antennas and other combinations of antennas. The second tuner assigns a quality measure to each of these signals. When the quality measure of the signal producing the viewed image is significantly inferior to some other input signal combination, as happens when the signal multipath interference environment changes due, for example to people moving about in the television viewing room, the television receiver changes the input signal for viewing to whatever other antenna, or particular combination of antennas, has the highest quality measure.